DescriptionThe study of aggressive behavior has received a great deal of attention in the development literature. Although it is clear that aggression appears early in development, and childhood aggression is a well-known predictor of future engagement in maladaptive behavior, research on aggressive behavior with preschool-aged children is still limited. Improving our knowledge about what motivates children to engage in aggressive behavior is vital in developing effective strategies and interventions aimed at reducing childhood aggression. In Study 1 we documented the incidence of physical, verbal, and relational acts of aggression in preschool-aged children. The results of Study 1 highlight the surprising complexity and context of aggressive behavior in this young age group. In Study 2, we successfully designed and implemented a new task aimed at assessing patterns of social information processing in preschool-aged children. Results of Study 2 demonstrate that problematic patterns of social information processing in preschoolers are associated with higher teacher ratings of proactive and reactive physical aggression. In Study 3 we examined how children's theory of mind abilities, along with their social information processing, contribute to young children's aggressive behavior. Results from Study 3 demonstrate that the relationship between patterns of social information processing and aggressive behavior differ as a function of children's theory of mind, with theory of mind performance mediating the relationship between children's patterns of social information processing and their aggressive behavior. More specifically, problematic responding to questions assessing patterns of social information processing, coupled with poor performance on select Theory of Mind items was related to higher levels of physical aggression. Likewise, appropriate social information processing patterns, coupled with good performance on select Theory of Mind items was related to higher levels of relational aggression.